I personally would start out with a 36. You can get thru the gates and it will mow larger accounts just takes more time. At first you will have this time. As things pick up put a bigger deck on the trailer. Think I would be looking at a walk behind for the smaller mower and as things evolve a larger ZTR

My neighbor bought one of the Scag 36" riders. He said it is a great machine, but that thing is like ride a bucking bronco. Every little bump was magnified because of the small wheel base. But, like I said that was the only down fall to it. Their faster than a walk behind unless your using a sulkie or velke.

your clientele should be what drives your equipment choices. As PM said, what sized lots do you cut? How many of those will have fenced rear yards? Of those that are fenced, what will be the smallest gate and how big will that lawn be in the fenced area. For economics, I can't think of a reason to buy a rider with a 36" deck. For that money, you could buy two walk behinds with sulkies and double your production. If you don't have any clients yet, you are putting the cart before the horse and may become part of the horde of equipment misfitted contractors out there bleeding off their profits with equipment they can't maximize.